As has been previously recognized, embedded silicon germanium (SiGe) technology has become a promising technology for producing silicon high-performance p-type field-effect transistors (PFETs). In particular, it has been shown that embedding SiGe in a silicon substrate next to a PFET channel causes compressive stress on the channel, thereby increasing hole mobility and increasing the performance of the PFET. This compressive stress property is discussed, for example, in an article entitled “35% Drive Current Improvement From Recessed-SiGe Drain Extensions on 37 nm Gate Length PMOS,” by P. R. Chidambaram, et al., 2004 Symposium on VLSI Technology, Digest of Technical Papers, pp. 48-49.
Referring to FIG. 23, which has been substantially reproduced from the above-mentioned reference article, it can be seen that, in general, channel stress is related to the relative distance of the SiGe layer from the channel. More particularly, part (a) of FIG. 23 shows a 30 nm deep layer of SiGe extending toward the channel only as far as the source/drain (SD) region. Part (c) shows the same 30 nm deep layer of SiGe extending toward the channel, but this time extending further (and thus closer to the channel) into the drain extension (DE) region. As can be seen in the graph of part (b), the stress at the center of the channel (distance=0) is approximately 250 MPa for the part (a) configuration, whereas the stress at the center of the channel is approximately 900 MPa for the part (c) configuration. Therefore, it can be seen that channel stress is increased by forming the SiGe layer closer to the channel.
Since compressive channel stress in a PFET is good in that it increases hole mobility in the channel, it would be desirable to be able to increase the channel stress even more. However, attempting to do so most likely results in substantial complications to be overcome, as will be discussed later. Accordingly, new techniques need to be developed to increase channel stress without substantially degrading extension junction characteristics.